Direct methanol fuel cells (DMFCs) using methanol as a fuel have been expected and increasingly developed for practical use as power sources for portable units, as a substitute for lithium ion secondary batteries.
Such DMFCs structurally include a cathode catalyst layer and an anode catalyst layer as electrodes; and a proton conductive solid polymer electrolyte membrane (proton-exchange membrane) is disposed between the cathode catalyst layer and the anode catalyst layer. This structure is known as a membrane electrode assembly (MEA). The cathode catalyst layer and anode catalyst layer each have a matrix containing a catalyst-supporting carbon and a solid polymer electrolyte in a suitable ratio, in which electrode reactions occur at a three-phase interface where the catalyst on the carbon, the solid polymer electrolyte, and a reactant are in contact with one another. The continuous carbon provides an electron transport path, and the continuous solid polymer electrolyte provides a proton transport path.
In DMFCs, reactions represented by following Schemes (1) and (2) occur in the anode catalyst layer and cathode catalyst layer, respectively, to deliver electricity.CH3OH+H2O→CO2+6H++6e−  (1)O2+4H++4e−→2H2O  (2)
DMFCs are believed to theoretically have energy densities about 10 times as much as those of lithium ion secondary batteries. Under present circumstances, however, their MEAs show still insufficient output as compared to lithium ion secondary batteries and have not yet been practically used.
To improve the output of MEAs, for example, the component catalysts and electrolyte membrane may be improved and/or the MEA structure may be optimized. Among them, improvements in electrolyte membrane is important to improve the output of MEAs effectively.
The electrolyte membrane is required to have a high proton conductivity and a low methanol permeability.
The high proton conductivity provides a low resistance in the electrolyte membrane, and the low methanol permeability prevents “crossover” in which methanol in the anode permeates through the electrolyte membrane and reaches the cathode. Methanol after reaching the cathode chemically reacts with oxygen on the cathode catalyst and generates heat. The crossover phenomenon causes increase of overvoltage in the cathode, resulting in decreased output of MEA.
The most popular electrolyte membrane now available is a perfluorosulfonic acid membrane supplied by DuPont under the trade name of Nafion (registered trademark). Nafion has a hydrophobic PTFE backbone and side chains including fixed terminal hydrophilic sulfonic acid groups. In a hydrous state, sulfonic acid groups, protons, and water molecules associate to form ion clusters. In the ion clusters, sulfonic acid groups are present in a high concentration so as to provide a proton transfer path, leading to a high proton conductivity. These ion clusters, however, also allow methanol to transfer therethrough, because methanol is miscible with water and movable with water. This causes a high methanol permeability. As is described above, Nafion is disadvantageous in its high methanol permeability, in spite of its high proton conductivity.
Examples of electrolyte membranes other than Nafion are hydrocarbon-based membranes and aromatic hydrocarbon membranes, each of which has a proton donor such as sulfonic acid group, phosphonic acid group, or carboxyl group. These electrolyte membranes exhibit their proton conductivity in a hydrous state so as to release protons, as in Nafion. It is possible to increase the proton conductivity by increasing the concentration of the proton donor such as sulfonic acid group. However, such a high concentration of the proton donor such as sulfonic acid group causes easy movement of water, and this in turn causes a higher methanol permeability.
As is described above, there is a trade-off between proton conductivity and methanol permeability in single organic polymer electrolyte membranes, and it has been difficult to provide electrolyte membranes having both a high proton conductivity and a low methanol permeability.
As a candidate for electrolyte membranes having both a high proton conductivity and a low methanol permeability, there have been received attention inorganic/organic composite electrolyte membranes including an inorganic material and an organic material as a composite. Typically, Materials Letters, 57 1406 (2003) refers to a composite electrolyte membrane that contains a polyvinyl alcohol as an organic material, and 12-tungstophosphoric acid, one of heteropolyacids, as an inorganic material dispersed in the polyvinyl alcohol. AIChE Journal, 49 991 (2003) refers to a composite electrolyte membrane which includes a polyvinyl alcohol as an organic material, and mordenite, one kind of zeolite, as an inorganic material dispersed in the polyvinyl alcohol. J. Membrane Science, 203 215 (2002) refers to a composite electrolyte membrane which includes a sulfonated polyether ketone or sulfonated polyether ether ketone as an organic material, and SiO2, TiO2, or ZrO2 as an inorganic material dispersed in the organic material.
Japanese Unexamined Patent Application Publication (JP-A) No. 2003-331869 discloses a composite electrolyte membrane which includes an organic polymer and a metal-oxide hydrate dispersed therein. Although being not an inorganic/organic composite electrolyte membrane, PCT International Publication Number WO 00/54351 discloses an electrolyte membrane which includes a porous base material that is not substantially swellable to methanol and water, whose pores are filled with a proton conductive polymer.
As is described above, composite electrolyte membranes have received attention as electrolyte membranes having both a high proton conductivity and a low methanol permeability.
The present inventors revealed that, of these composite electrolyte membranes, those including a proton conductive metal-oxide hydrate and a proton conductive organic polymer are a potential candidate for electrolyte membranes having both a high proton conductivity and a low methanol permeability. It was expected that such a metal-oxide hydrate blocks methanol and allows selective transfer of protons; and that composite electrolyte membranes including the metal-oxide hydrate dispersed in an organic polymer can have both a high proton conductivity and a low methanol permeability.
Even these composite electrolyte membranes, however, should satisfy various conditions in order to exhibit their original characteristic properties sufficiently.
Accordingly, an object of the present invention is to provide both a high proton conductivity and a low methanol permeability in a composite electrolyte membrane including an organic polymer and a metal-oxide hydrate dispersed therein, and to improve the output of an MEA.